The automation of book binding has allowed book production to proceed as an essentially continuous process from the printing and compilation of the pages into book blocks, through the gluing of the block spines, to the application of the cover stock and trimming. In the gluing step, a water-based primer composition comprising a polymeric adhesive is applied to the spine area of the book blocks, which may be in the form of pre-glued or sewn individual signatures. The coating of wet glue is next dried and set to form a flexible film by exposure of the coating to an intense, high temperature heat source. At least one coating of a covering adhesive is then applied over the primer layer to provide the backbone, the cover is adhered to the backbone and the bound pages are trimmed.
The maximum speed at which the adhesive-application steps could be accomplished has been limited by the time needed to dry and set the water-based primer prior to application of the covering adhesive layer, which may be a hot-melt type adhesive. If the primer coating is not sufficiently dried, application of a hot-melt adhesive can cause splattering and blistering of the aqueous primer adhesive. On the other hand, the exposure of the water-based primer layer to the extreme temperature gradients required for high speed drying often causes substantial problems. Conventional primers skin over when exposed to intense heat sources; the wet coating becomes surfaced with a thin polymeric membrane. The water entrained in the interior of the primer layer vaporizes and lifts this dried membrane into large blisters which break and burn as they approach the heat source. If the subsequently-applied adhesive fails to adhere fully to these blistered or burned primer layers, the binding process cannot be completed satisfactorily. The damaged, partially-bound books tend to jam in the assembly line and must be removed and discarded, resulting in substantial economic losses.
Therefore a need exists for improved water-based primer adhesives which resist blistering and charring when exposed to high drying temperatures.